Hello and welcome!

Honestly I think classes and specs are down to preference. Just about everything is doable to an extent, but some classes tend to yield better efficiency in a multibox setup than others. For example hunters have been the go-to ranged DPS for a long while if you're looking for the "best" output in a mixed setup. The general rule of thumb is that proc based specs are commonly worse for us, as they require more individual baby sitting and attention.

It's often recommended to start with 5x (or however many) of the same class and spec, since it's generally simpler for a beginner. I don't think a mixed setup is beyond the grasp of a beginner either, but it takes more setup and fine tuning, so for those just wanting to get going on their adventures "all of X" is usually a good starting point. Again it's mostly down to what you want to play, but it's generally agreed that different roles have varying degrees of difficulty when multiboxed. From easiest to hardest you'd be looking at something like:

Ranged DPS
Ranged DPS pet class
Melee
Mixed groups

Looting can be made easier with custom mapped keys that utilize the Interact with Target function. Here's a video of Mirai demonstrating his loot method.